1. Field of the Invention
The present invention relates to a color display device which employs an electroluminescence (hereinafter referred to as EL) self-light emitting element and a thin film transistor (TFT) and further to a method of laying out such a device.
2. Description of the Related Art
In recent years, EL display devices which use EL elements have come to attract a great deal of attention as possible replacements for CRT and LCD display devices. As a means for achieving color display using such EL display devices, a number of methods have been suggested, including an RGB color discriminating method which uses light emissive materials to provide light emission of the three primary colors red, green, and blue, and a method which uses a color filter or a color conversion layer relative to monochrome light emissive materials.
In a color discriminating method, different light emissive materials, each of which is unique in characteristics, such as chromaticity, a service life, light emission efficiency, and so forth, are used for different colors. Generally, in order to maintain appropriate white balance, display devices for color image displaying automatically determine required light emission luminance based on the chromaticity of the light emissive materials used for the respective colors. As the luminance is substantially proportional to the density of a current supplied to the light emissive material, a light emissive material with poor light emission efficiency requires a larger current density in order to obtain the required light emission luminance. However, increased current density places a burden on the material and leads to reduction of the service life of the material and, thus, of the EL display device itself.
FIG. 16 is a plan view schematically showing an organic EL display device disclosed in EP1032045A2, and proposed to address the above problem. In this device, regions each enclosed by a gate signal line 51, a drain signal line 52, and a driving power source line 53 are arranged in a matrix, and light emitting regions R90, G90, and B90 for respective colors, each having a different area depending on a color, are arranged each in each enclosed region. In the drawing, the light emitting regions R90, G90, and B90 each represent a region with visibly recognizable light emission, and letters R, G, and B represent the colors red, green, and blue, respectively.
One key criterion for determining the light emitting area for each different color is the light emission efficiency of the light emissive material used for that color. That is, a light emitting region made of a light emissive material with a relatively poor light emission efficiency is formed having an area larger than that of regions made of other materials in order to obtain the desired light emission luminance.
In this manner, this arrangement can prevent a current of excessive density from being supplied to a light emissive material with poor light emission efficiency, and therefore extend the service life of such a material. However, because an interval of signal lines and of driving lines are determined so as to accommodate the requirement of a color which requires the largest light emitting area, as shown in FIG. 16, pixel spaces are not efficiently utilized in this device, and the device is therefore unsuited for use in a high-density structures.